It’s called Kharfun Syndrome, and it plays a large role in the history of the Confederation. It first arose among Humans, for whom it was a flu-like but usually survivable disease. Many children got it, developed immunity, and went on to lead normal lives. But it became endemic in the Human population.

The early symptoms are mild – aches and pains, some muscle twitches – and that was as far as it got with a good functioning immune system. For those whose immune systems could not handle it, the virus gradually attacked the peripheral motor nerves, leading to violent muscle cramps which was followed by paralysis, and eventual death from respiratory paralysis. The peripheral sensory nerves were also involved during the active phase, with pain spreading inward from the fingers and toes.

The Human immune system, which is basically chemical in nature, could handle the virus. I’m not going to go into the full immune system here, and in fact there’s a lot we don’t know about it. But there are times when it goes wrong and attacks something it shouldn’t. Like the Islets of Langerhans in my pancreas (which is why I have type 1 diabetes) or the myelin sheaths of my sister’s nerves (Multiple Sclerosis.) Perhaps because of this the R’il’nai, who have a suite of esper abilities and could actually perceive bacteria and viruses and remove them without even being consciously aware of the process, developed an immune system based on esper, and the old-chemical-based system, while still present, became very inefficient.

The problem with Kharfun was that the virus causing it had evolved an ability to hide from esper perception.

As a result, Kharfun was originally 100% lethal to those whose immune systems relied on esper – all pure R’il’nai, and most of the hybrids with a large fraction of active R’il’nian genes. A method of reactivating the old, chemical-based immune system was developed after the disease spread from Humans to R’il’nai, but by that time a large fraction of the R’il’nai had died.

The disease had another effect on the R’il’nai – it reduced their already low fertility. They didn’t have a high birth rate to start with – R’il’nian females were fertile for a few hours a century. (They were usually receptive, but not fertile.) And the immunization had the same effect as the disease on fertility.

So 10,000 years after the initial epidemic, the R’il’nai are nearly extinct. This was the premise behind Homecoming (where Kharfun Syndrome plays a major role) and the society that led to Tourist Trap and the trilogy I’m working on.